Recent development and widespread use of medical image devices for computed tomography (CT) and magnetic resonance imaging (MRI) have made it possible to obtain a large volume of high-definition digital images for medical use. Such medical images are accumulated in picture archiving and communication systems (PSCS), together with findings (report) obtained when a doctor examines an image for diagnosis. The amount of medical images which can be acquired has increased more and more with the enhanced performance of devices, and thus a medical image compression technique which allows high compression is in high demand. In addition, a high image quality is required for medical images, and thus generally lossless image compression is carried out. General compression techniques such as JPEG-LS, Lossless JPEG, and JPEG2000 are used in current medical image compression. In recent years, in addition to these technical backgrounds, images of tissues or cells along with radiological images are also stored increasingly as virtual slides. These images have larger volume compared to the radiological images, and thus a demand for medical image compression is likely to increase more and more in the future.
Patent Literature (PTL) 1 discloses a conventional technique related to the lossless medical image compression. PLT 1 focuses on the point that medical images have larger noises compared to natural images, and presents a solution for that. In particular, in the case of image capturing using multi-slice CT, radial noise (artifact) is likely to be generated, leading to decrease in the compression efficiency. PLT 1 solves such a problem by dividing an image into regions and performing compression for each corresponding region, improving the compression efficiency in conventional lossless compression on medical images.